This invention relates to an analog fluidic speed sensor and method. More particularly, this invention relates to a speed sensor providing a fluid pressure signal which is analogous to the velocity of a moving surface. The sensor provides a fluidic speed signal having a comparatively high signal-to-noise ratio despite the presence of a turbulent fluid boundary layer adjacent the moving surface.
A conventional fluidic speed sensor is known including a fluidic amplifier having a housing rotatably an input shaft. The fluidic amplifier includes a pair of oppositely disposed arcuate control passages respectively extending from a pair of control inlets to a power nozzle. The input shaft drivingly carries a disc defining a planar surface bounding the pair of control passages. Consequently, fluid flowing in the control passages is influenced by rotation of the input shaft and disc. According to the influence of the disc upon the fluid flowing in the control passages, a stream of fluid issuing from the power nozzle is deflected with respect to a splitter. The splitter separates a pair of output legs communicating with respective outlet passes. U.S. Pat. No. 3,528,298, granted Sept. 15, 1970 to E. G. Zoerb illustrates a conventional fluidic speed sensing apparatus of the above-described type.
A conventional speed sensor according to the Zoerb invention has a number of recognized deficiencies. Among these recognized deficiencies is the fact that the housing of the sensor must define a bore rotatably receiving the input shaft. Consequently, additional manufacturing steps and expense are necessary to provide the bore. Further manufacturing steps may be required to fit low-friction bearings into the bore for journaling the shaft. Yet another recognized deficiency of the Zoerb invention is that the input shaft and disc are integral parts of the speed sensor. In many situations where it is desired to measure the rotational speed of a shaft, it is difficult or impossible to drivingly connect the rotating shaft to the input shaft of the sensor. For example, it may be necessary to provide a gear on the rotating shaft to couple with a gear on the input shaft of the speed sensor. Another conventional expedient is to provide the input shaft of the sensor with a friction wheel engaging the outer surface of the rotating shaft. Both of these conventional expedients, and others, for coupling the speed sensor to a rotating shaft involve the provision of special gearing, shafting, or other driving apparatus coupling the sensor to the rotating shaft as well as the provision of mounting structure for holding the speed sensor adjacent the rotating shaft in proper relation to the driving apparatus. All of these provisions involve additional expense in the use of a speed sensor according to the Zoerb invention. Further, in those frequent situations where space for the sensor is limited, the necessary driving apparatus and mounting structure may prohibit altogether the use of a sensor according to the Zoerb invention.
Another conventional fluidic sensor includes a fluidic amplifier disposed adjacent to the disc of a gyroscope. The amplifier housing defines a part-spherical recess leading to a circular aperture. The circular aperture opens to the interaction region of the fluidic amplifier. The rim of the gyroscope disc is received in the part-spherical recess so that the outer circumferential surface of the disc is substantially tangent to the power jet of the amplifier. When the gyroscope disc rotates, a fluid boundary layer forming adjacent to the rim interacts with the power jet in the interaction region of the amplifier to effect a deflection of the power jet resulting in a fluidic output signal. Thus, the fluidic sensor produces an output signal analogous to the position of the gyroscope disc. U.S. Pat. No. 3,311,987, granted Apr. 4, 1967 to H. Blazek illustrates a fluidic sensor of the above-described conventional type.
A fluidic sensor according to the Blazek reference also has a number of recognized deficiencies. For example, the circular aperture opening to the interaction region of the amplifier is comparatively large in relation to the size of the interaction region. Because the power jet of the amplifier may be deflected anywhere along its course from the nozzle to the splitter, the comparatively large aperture may result in the jet being deflected a number of times as the jet traverses the aperture. As a result, the output signal of the sensor may not vary linearly with varying speed of the gyroscope disc. Further, when the fluid boundary layer is turbulent, the power jet is exposed to the turbulence over a comparatively long distance so that considerable noise may be induced into the output signal by the turbulence.
Furthermore, both the speed sensor of Zoerb and the sensor of Blazek suffer from yet another recognized deficiency. This deficiency stems from a natural tendency for fluid boundary layers to become turbulent above a certain speed, as indicated by Reynolds number. Below the certain speed, the fluid boundary layer is laminar and smoothly flowing. Consequently, the laminar boundary layer may interact with the power jet of a fluidic amplifier without inducing noise in the output signal of the amplifier. However, above a certain speed the boundary layer becomes turbulent and superimposes noise upon the fluidic output signal of the amplifier. This noise decreases the accuracy of the speed indication which is obtainable from the amplifier. The particular sensitivity to turbulence-induced noise of a sensor according to the Blazek invention has been pointed out supra.